Regenerable filter elements are used to collect and burn soot particles, in particular, carbon particles from diesel engine exhaust. In order to meet pollution standards for diesel engines, the filter elements are an important component of the exhaust filters. These elements are capable of being regenerated in that the soot particles contained within the elements can be burned off in situ, allowing the return of the element to its original soot-free state. This burning is generally initiated by means of a glow plug which is a component in the exhaust filter to ignite the soot captured within the filter element. Once ignited, the soot continues to burn on its own, and in so doing causes the filter element to reach very high temperatures. Eventually, the soot burns itself off and the filter element cools down to a normal operating temperature. The filter element continues to collect soot and so eventually will need to be regenerated again in the above described manner. Therefore, the burning off of the soot is done on a periodic basis, either after a predetermined number of hours of operation, or when the pressure drop through the filter due to soot build-up reaches a predetermined level.
Conventional diesel exhaust filter elements are made of porous structures of cordierite or other materials that are capable of trapping the small soot particles from diesel exhausts while allowing the exhaust gases themselves to flow through the elements. In order to form the filter element, these materials are fashioned into various structures such as a reticulated ceramic foam, or a mat of bonded fibers. These materials are then formed into configurations that greatly increase the surface area of the element. Traditionally, this was done by configuring shapes out of the elements that direct the exhaust gases down long narrow channels that are plugged at their downstream ends, forcing the exhausts to seep through the walls of the channels where the soot is trapped. Once on the other side of the channel walls, the exhaust gases are now in channels which are plugged at their upstream ends and the gases are forced out the downstream end of the filter element. One of the usual methods for forming the channels was by fabricating corrugated sheets bound on both sides by flat sheets. These resultant channels were alternately plugged, appropriately, at one end or the other and the bound sheets were rolled into a spiral configuration to form a cylindrical filter element. In addition, structures such as a bundle of hollow tubes with a honeycomb or some other polygonal cross-section were also utilized.
These previous elements were limited in that they lacked: 1.) the strength needed to endure the ride of a moving vehicle, 2.) the ability to stand-up to the high temperatures achieved during the burning off of the soot, and 3.) the endurance to withstand the numerous thermal cycles resulting from the repeated regeneration process that is to be encountered throughout the lifetime of the vehicle. It is not desirable to have to replace the elements due to the added expense and the inconvenience. Cordierite in particular is not very attractive for these reasons, as it is not nearly as strong as mullite and it also has a lower service temperature (1000.degree. C.) than does mullite (1700.degree. C.). Cordierite also has a lower heat capacity per unit volume than does mullite. A lower heat capacity is undesirable because it results in the filter element reaching a higher temperature during regeneration and this adds to degradation of the element due to the thermal shock experienced by the element.
Considerable pressure drop was experienced across the above described prior art elements, that is to say, a reduction in pressure from the input of the element to the output of the element. This is a consequence of the exhaust gas passing through a tortuous path of narrow pores in the elements. This pressure drop must be kept to a minimum as it results in back pressure being experienced by the engine which degrades the performance of the engine. Pressure drops in these types of filter elements are affected by such factors as the surface area of the element, the permeability of the element, and the amount of soot presently collected within the element. Generally, the greater the surface area of an element, the less the pressure drop across the input and output of the element under a given set of operating conditions. The same generally holds true for greater permeability. Furthermore, an increase in soot build-up increases the pressure drop, so it would be especially advantageous to have a filter element with a lower pressure drop.
Prior art attempts to solve these problems have included: (1) forming cordierite into a spiral corrugated form; (2) a mat of bonded fibers formed from various ceramic materials; and (3) a bundle of hollow tubes with a honeycomb cross-section also formed from various ceramic materials. Specific examples of these attempts are as follows.
U.S. Pat. No. 4,560,478 issued to Narumiya on Dec. 24, 1985 and assigned to Bridgestone Tire Co. of Tokyo discloses a porous ceramic article comprising a porous ceramic body having a three-dimensional network of strands defining interconnected cells. The cells are characterized in that at least one compound selected from the group consisting of nitrides, carbides, borides and silicides of metals is dispersed in or deposited on the strands of the porous ceramic body. It is stated in the patent that the porous ceramic articles or structures may be used as traps for particulates in exhaust gases such as diesel engine exhaust gases, or for other filter applications. It was furthermore stated that the porous ceramic bodies are preferably formed of needle-like crystals of mullite formed from alumina or cordierite to achieve high temperature strength, and that the body is preferably formed of these materials by applying a ceramic slurry to an open-cell foam of synthetic resin to form a reticulated ceramic foam structure.
U.S. Pat. No. 4,264,760 issued to Higuchi et al. on Dec. 21, 1982 and assigned to NGK Insulators, Ltd. of Nagoya, Japan discloses a ceramic honeycomb filter comprising a ceramic honeycomb structural body having a multiplicity of parallel channels extending therethrough. The selected channels are sealed at one end of the body while the remainder of the channels are sealed at the opposite end of the body in such a manner that, as dust-containing gas flows therethrough from one end of the body to the opposite end of the body, the gas flows through the walls between adjacent ceramic channels where the dust particles are collected. The production of ceramic honeycomb structural bodies was furthermore described as an extrusion process starting with fine powders of raw material such as alumina, silica, mullite, silicon carbide, silicon nitride, cordierite or the like blended with an organic binder and a plasticizer. The mixture is extruded through a die having a large number of slits capable of forming channels of a given shape in the monolith structure to be extruded. The extruded structure is dried and fired to obtain a porous ceramic honeycomb structural body.
U.S. Pat. No. 4,652,286 issued to Kusuda et al. on Mar. 24, 1987 and assigned to Matshushita Electric Industrial Co., Ltd. of Kadoma, Japan discloses an exhaust gas filter for diesel particulates, comprising a row of a plurality of channels of a honeycomb structure of porous sintered ceramic fiber composite sheets. The ceramic fiber composite sheet is produced by a paper-forming method from a slurry of alumina-silicate fibers and fire clay. The honeycomb structure of the ceramic fiber composite sheet is formed by stacking planar sheets and corrugated sheets one atop the other. The production of the sintered ceramic fiber composite sheet was furthermore described as starting with ceramic fiber of alumina-silica or silica which is cut by a chopper into short fibers or predetermined lengths. Further steps are taken to form a slurry which is eventually sintered to yield the ceramic fiber composite sheet.
U.S. Pat. No. 4,761,323 issued to Muhlratzer et al. on Aug. 2, 1988 and assigned to Man Technologie GmbH of Munich discloses a method for the production of soot filters using felt-like or other bats as filter elements which are made up of loose refractory fibers. To bond the fibers together and to anchor them in place, the filter element is coated by chemical vapor deposition or precipitation from a solution to give an amorphous, refractory coating which bonds the fibers together at their crossovers. The starting material was furthermore described as a felt-or wadding-like fiber bat made up of loose fibers which are mixed and entangled with each other.
PCT/US 89/03175 to Talmy, Inna G. and Haught, Deborah A. filed on 26 July 1989 discloses a process in which AlF.sub.3 and SiO.sub.2 or AlF.sub.3, SiO.sub.2, and Al.sub.2 O.sub.3 powders are formed into a green body of a desired shape and size and then heated at 700.degree. C. to 950.degree. C. in an anhydrous SiF.sub.4 atmosphere to form bar-like topaz crystals, then heated in an anhydrous SiF.sub.4 atmosphere at about 1150.degree. C. to 1700.degree. C. to convert the bar-like topaz to needle-like single crystal mullite whiskers which form a porous, rigid felt structure. The felt structure was described as comprising single crystal mullite whiskers which were uniformly distributed and randomly oriented in three dimensions and which were mechanically interlocked to form a rigid felt structure capable of maintaining its shape without binders. The whiskers were composed of stoichiometric mullite or solid solutions of Al.sub.2 O.sub.3 in stoichiometric mullite. A suggested application for the felt was as preforms for ceramic-matrix or metal-metal matrix composites or by itself as thermal insulation.
Therefore, it is an object of the present invention to provide a regenerable filter element which is more permeable, stronger, more tolerant of high temperatures and more tolerant of thermal cycling than prior art elements. This element may include an interlaced network of mullite crystals that are grown and fused together to form a rigid porous body for the purpose of collecting and burning off soot from diesel engine exhaust.
It is yet still a further object of the present invention to provide a regenerable filter element which includes an interlaced network of mullite crystals that are formed and fused together by pyrolysis of fluorotopaz to form a rigid porous body for the purpose of collecting and burning off soot from diesel engine exhaust.
It is yet still a further object of the present invention to provide a regenerable filter element which includes an interlaced network of non-stoichiometric mullite crystals of about 2:1 molar ratio of alumina to silica that are formed and fused together to form a rigid porous body for the purpose of collecting and burning off soot from diesel engine exhaust.